The present invention relates to a water-based photopolymerizable resin composition. More Particularly, the invention relates to a water-based photopolymerizable resin composition which can be used in the preparation of a photosensitive resinous printing plate having excellent pliability and durability in printing and free from the problem of crack formation even when the printing plate is used on a printing press with a cylinder of small diameter under a cold and low-humidity condition.
Known photosensitive resin compositions in the prior art used for the preparation of relief printing plates are classified depending on the solubility behavior into those soluble in an aqueous alkaline solution, those soluble in an organic solvent such as alcohol and those soluble in neutral water. As a trend in recent years, the last mentioned water-soluble ones are used increasingly in view of the advantages in the safety of processing works with the same, absence of the problem on the workers' health and absence of the problems relative to the waste disposal.
Several different types of water-soluble photosensitive resin compositions are known for use in the preparation of printing plates including those containing a poly(vinyl alcohol), an acrylamide compound and a photosensitizer disclosed in Japanese Patent Publication 39-25941, those containing a graft-copolymerized poly(vinyl alcohol) and a photosensitizer disclosed in Japanese Patent Publication 44-28275, those containing a poly(vinyl alcohol) esterified with an unsaturated carboxylic acid as the photosensitizing ingredient disclosed in Japanese Patent Publication 48-6962, those containing a combination of a poly(vinyl alcohol) and a diazonium salt or tetrazonium salt as the photosensitizing ingredient or those containing a combination of a poly(vinyl alcohol) and an acrylate compound such as 2-hydroxyethyl methacrylate disclosed in U.S. Patent No. 3,801,328, and so on.
These prior art water-based photosensitive resin compositions in general, of which the characteristic ingredients are a poly(vinyl alcohol) and a constituent having a photoreactive group, have disadvantages of low developability in patterning, poor versatility of the starting materials to satisfy specific requirements for the properties and difficulty in the purification of the photosensitive constituents although the printing plates prepared therefrom have good resistance against water and chemicals and adequately high hardness. Since poly(vinyl alcohols) or derivatives thereof and unsaturated compounds conventionally used as a photosensitive ingredient are poorly compatible with each other, printing plates having sufficiently high hardness and water-resistance can be obtained with difficulties and cracks are sometimes formed in the printing plates prepared using the composition, especially, under a cold and low-humidity condition.
Alternatively, Japanese Patent Publication 53-2082 proposes a water-based photosensitive resin composition comprising a water soluble nylon resin and a photosensitizing agent. This composition, however, is not practical because, in addition to the poor compatibility of the water-soluble nylon resin and the photosensitizinq aqent, an insoluble matter is sometimes precipitated in the depleted developer solution and the precipitates must be dissolved away by adding an acid such as acetic acid to the solution in order to prevent cloqqinq in the pipe lines.
With an object to solve the above mentioned problems in the water-soluble photosensitive resin compositions in the prior art, the inventors have previously proposed a water-soluble photosensitive resin composition comprising a poly(vinyl alcohol) and a polycondensate having compatibility therewith, such as a polycondensate of an alkylol urea and an N-alkylol acrylamide or a polycondensate of urea and formaldehyde modified by the addition-condensation of an N-alkylol acrylamide (see Japanese Patent Publication 54-3790). Although the Printing Plate Prepared from this composition is excellent in general properties, a problem thereof is that the relief prepared by plate making is sometimes too hard and subject to crack formation so that improvement in this regard is eagerly desired.
The inventors further proposed, in Japanese Patent Kokai 60-101531, a photosensitive resin composition comprising a water-soluble resin, photopolymerization initiator and reaction product of a polyglycerin and a photopolymerizable ethylenically unsaturated compound. Although the problem of crack formation in the printing plate prepared therefrom can be solved to a considerable extent, the improvement is still insufficient so that crack formation in the printing plates cannot be completely prevented especially when the printing plate is used on a printing press having a small-diameter cylinder such as a label printing machine.